1. Field of the Invention
The present invention relates to a sheet thickness measurement method, swell measurement method and an apparatus for performing such measurements. Such apparatus and method are employed during the manufacturing process of various types of film and sheet, such as those made of synthetic resin, metal, paper, wood, glass, ceramic, etc., laminate films, coating membrane, and sheet type food (collectively referred to as xe2x80x9csheetxe2x80x9d hereafter), to measure the thickness or swell of the sheet being manufactured. The measured thickness and/or swell are displayed and recorded to supervise the manufacturing processes, and/or to correct deviation from target thickness or target swell value.
2. Description of the Prior Art
A conventional sheet thickness measurement apparatus is equipped with a main frame having an aperture for allowing a sheet to pass therethrough; a pair of measurement heads having a pair of sheet measurement sensors; and a moving mechanism for moving the pair of measurement heads in a width direction perpendicular to a direction of the sheet passing. The conventional sheet thickness measurement apparatus measures the sheet thickness by the above-mentioned pair or sheet thickness measurement sensors, and displays the results.
However, in the manufacturing process of sheets, radiation heat, friction heat or the like causes thermal deformation and distortion, such as warp and bend, of the main frame of the measurement apparatus and guiding members of the moving mechanism for each measurement head of the measurement apparatus, which results in deterioration in measurement accuracy.
As a method for solving this problem, periodic calibration of the reference value of the measurement sensor (the zero point or the thickness of a reference, such as a master gauge, for example) has been known.
This calibration of the reference value is conducted as follows. A pair of measurement heads is moved to a position outside of the. measurement region of a sheet. Then, the pair of measurement sensors is approached until they are directly contacted to each other to perform the calibration. Alternatively, the pair of measurement sensors is indirectly contacted to each other through a reference thickness part, such as a master gauge, for example. To explain more specifically with an example, suppose that thermal deformation of the main frame occurred between the time of the previous calibration of the reference value and the time of the present calibration of the reference value. In this case, when the thickness of a master gauge is measured through the pair of measurement sensors by approaching the pair of measurement sensors relative to the master gauge, the resulting measured thickness differs from the previously measured value by an amount corresponding to the amount of the thermal deformation of the main frame. A measurement system is thus calibrated such that the measured value of the master gauge becomes a new reference value, and the measurement operation for the sheet thickness is thereafter resumed. On the other hand, in the case where the pair of measurement sensors is directly contacted to each other for the calibration instead of using a master gauger the zero point would be calibrated.
However, the methods for calibrating the reference value, as described above, have the following drawback: Because the calibration operation is performed at a position outside of the actual measurement region of a sheet, thermal deformations of the main frame and moving mechanism for each measurement head within the measurement region of the sheet cannot be comprehended. This problem is caused by the fact that the master gauge, which forms a reference plane for the measurement sensors, cannot be placed along the passage of the sheet, and leads to a serious problem of degradation in reliability of measurement accuracy in the technical fields that require high accuracy measurement of the sheet thickness in the order of micron.
In the conventional art, avoidance of the above-mentioned problem requires improvement in linear precision of a moving guide part of a guide member, such as a guide bar, of the moving mechanism of each measurement head, elimination of looseness on the sliding surfaces of the guide bar and the measurement head, and employment of technique that dissipates thermal deformation of the guide bar, etc. However these countermeasures invite high costs in manufacturing the apparatus.
Accordingly, an object of the present invention is to realize accurate and on-demand calibration of a reference value within the measurement region of a sheet by a cheaper means.
Another object of the present invention is to provide a simpler configuration of means for creating a characteristic map for the reference plane (calibration map of the reference values for the entire measurement region of a sheet) and means for calibrating the map.
A sheet thickness measurement method according to the present invention includes, as a prerequisite, the steps of moving a pair of measurement heads in a direction of a sheet width; measuring the thickness of a sheet by a pair of sheet thickness measurement sensors installed on the measurement heads; and calibrating the measured values of the sheet thickness by a preliminary prepared characteristic map of the measurement heads with respect to the direction of the sheet width. In particular, the present invention is characterized by use of highly directional electromagnetic waves, such as light or beam, as a reference plane for creating the characteristic map.
Specifically, the method is characterized as follows: The highly directional electromagnetic waves, such as light or beam, which become the reference plane of the characteristic map, are emitted from one side of a main frame in a direction substantially parallel to a moving direction of each measurement head. Non-contacting position detection means is installed on a part of each measurement head or each sheet thickness measurement sensor, and each non-contacting position detection sensor receives the above-mentioned highly directional electromagnetic waves, such as light or beam, periodically, or continuously or on demand during the measurement operation, to detect displacements of the respective measurement heads relative to the above-mentioned reference plane. The characteristic map is then calibrated by these detected values.
According to the above-mentioned construction, it is possible to conduct accurate and on-demand calibration of the characteristic map of the reference plane. Further, it becomes possible to have a simpler construction of means for creating the characteristic map of the reference plane and its calibration means.
In the sheet thickness measurement method mentioned above, when a contacting sheet thickness measurement sensor is used, reference values are calibrated by having the above-mentioned pair of sheet thickness measurement sensors approaches and contacts to each other or to a master gauge periodically, or continuously or on demand during each measurement operation, at both ends of the moving range of the pair of measurement heads.
In the sheet thickness measurement method mentioned above, when a non-contacting sheet thickness measurement sensor is used, reference values are calibrated by having the above-mentioned pair of sheet thickness measurement sensors approaches to a master gauge periodically, or continuously or on demand during each measurement operation, at both ends of the moving range of the pair of measurement heads.
The above-mentioned calibration is performed at the time of creating the characteristic map and at the time of correcting the characteristic map.
In another aspect, a sheet thickness measurement method according to the present invention includes the steps of moving a pair of measurement heads in a direction of a sheet width; measuring the thickness of a sheet by a pair of sheet thickness measurement sensors installed on the above-mentioned measurement heads; concurrently therewith receiving, by non-contacting position detection means installed in each measurement head, directional electromagnetic waves, such as light or beam, that are emitted from one side of a main frame in a direction substantially parallel to a moving direction of each measurement head to detect displacements of each measurement head in a direction of the sheet thickness relative to a reference plane defined by the above-mentioned highly directional electromagnetic waves; and calibrating the measured values of the sheet thickness by measurement sensors installed on the above-mentioned measurement heads directly in accordance with the displacements of each measurement head from the reference plane, which are detected by the above-mentioned non-contacting position detection means.
According to this method, the measured value of each measurement sensor is directly calibrated without a need of the characteristic map. Thus, it becomes possible to monitor thermal deformation and the like in real time and to measure the sheet thickness with improved accuracy.
In still another aspect, a sheet thickness measurement apparatus according to the present invention includes, as a prerequisite, a main frame having an aperture for passing a sheet therethrough; a pair of measurement heads having a pair of sheet thickness measurement sensors; and a moving mechanism for moving the above-mentioned pair of measurement heads in a width direction perpendicular to a direction of the sheet passing, wherein the above-mentioned pair of sheet thickness measurement sensors measures a sheet thickness, and the measured values of the sheet thickness are calibrated by a characteristic map of the moving mechanism of each measurement head with respect to a direction of the sheet width. The sheet thickness measurement apparatus according to the present invention is further characterized by comprising highly directional electromagnetic waves emitting means installed in one side of the main frame, for emitting, from one side of the main frame, highly directional electromagnetic waves, such as light or beam, which become a reference plane for creating the characteristic map, in a direction substantially parallel to a moving direction of each measurement head; and non-contacting position detection means, installed in a part of each measurement head or part of each sheet thickness measurement sensor, for detecting changes in position of each measurement head in a direction of the sheet thickness through receiving the above-mentioned highly directional electromagnetic waves, such as light or beam.
The sheet thickness measurement apparatus of the present invention may be further characterized in that the means for emitting the highly directional electromagnetic waves towards each of the non-contacting position detection means comprises one highly directional electromagnetic waves emitting means and means for splitting highly directional electromagnetic waves, such as light or beam, emitted from the above-mentioned one highly directional electromagnetic waves emitting means.
The sheet thickness measurement apparatus above may be further characterized in that the installation position of each non-contacting position detection means is along a measurement axis of the corresponding sheet thickness measurement sensor or adjacent thereto.
The sheet thickness measurement apparatus of the present invention may be further characterized in that the non-contacting position detection means includes deflecting means for deflecting and receiving the highly directional electromagnetic waves, such as light or a beam, emitted from the highly directional electromagnetic waves emitting means.
The sheet thickness measurement apparatus of the present invention may be further characterized in that the highly directional electromagnetic waves emitting means emits parallel light rays having a single optical axis. The sheet thickness measurement apparatus of the present invention may be further characterized in that the highly directional electromagnetic waves emitting means emits parallel light rays having a plurality of optical axes parallel to each other.
The sheet thickness measurement apparatus of the present invention may be further characterized in that the highly directional electromagnetic waves emitting means is a laser light generator.
The sheet thickness measurement apparatus of the present invention may be further characterized in that the non-contacting position detect ion means is one-dimensional position detection means that is in parallel with the direction of the sheet thickness.
The sheet thickness measurement apparatus of the present invention may be further characterized in that the non-contacting position detection means is two-dimensional position detection means including the direction of the sheet thickness.
The sheet thickness measurement apparatus of the present invention may be further characterized in that the non-contacting position detection means is optical position detection means.
In a further aspect, a sheet swell measurement apparatus according to the present invention is characterized by measuring a swell of one side of a sheet by disposing highly directional electromagnetic waves emitting means and non-contacting position detection means on one side of the sheet. In particular, the measurement of the swell may be conducted using the sheet thickness measurement apparatus of the present invention on only one side; however, it is preferable to install the apparatus on both sides and use only necessary one without using the other unnecessary one*